The metal halide lamp has been found to be an efficient light source for use in growing plants indoors. The Westinghouse Electric Corporation offers a metal halide lamp for sale which provides an initial efficacy of 125 lumens per watt. A 1000 watt version of this lamp is said to produce more light output than three 1500 watt incandescent lamps or two 1000 watt mercury lamps. Moreover, the light spectrum produced by the lamp is highly satisfactory for plant growth.
If a reasonable return is to be realized from an investment in lamps of this type, they must be applied effectively. Ideally, the application should simulate as nearly as possible the natural conditions under which plant growth is sustained in nature.
A number of points may be noted concerning natural conditions of lighting as afforded by the sun. First, most plants require for at least a part of the day the full intensity of the sun's rays as experienced when the sun passes overhead. During the remainder of the day as the sun rises and sets, varying levels of intensity are acceptable, including periods of complete darkness. Secondly, it is desirable that the position of the light source should not be fixed relative to the plant. The rising and setting of the sun in the natural plant environment permits exposure of the lower leaves and penetration of light to the interior of the plant structure as the angle of incidence changes. Finally, the rotation of the earth permits the utilization of the single light source (the sun) as a means for sustaining growth all over the earth.